There isn't a way to create an Opt-Out system for a mailing not sent through Emma. The Opt-Out link that we provide in our emails is personalized for each email address as we send it out, so that it will come back to us in a trackable way.

If they have their own opt-out link that you can use to collect opt-out email addresses, you can then import that list into Emma and manually opt-out those addresses to ensure you don't send to them again.